Carbon Monoxide Detector Proper Placement

Interesting--I worked as a contractor in Vietnam and lost 2 of my employees to carbon monoxide. A pump in a well stopped working and the foreman went down in it, about 40ft, to see what wrong and the workers above saw him fall into the water and another one went down. He to fell in the water. When they were got out it was determined that the carbon monoxide had killed them. I had been down in that well myself no problem but we in the meantime had started a diesel power station right above the well. We were told that carbon monoxide had settled in the well because it was a heavier gas. Were we told wrong? I have always believed that since then.